This invention relates to a method for the production of a direct positive image and to a radiation sensitive element for use therein.
A number of methods for obtaining latent positive images are known. Thus for example United Kingdom patent specifications Nos. 1,151,310 and 1,376,826 require the use of a material comprising a combination of thin evaporated layers of a metal compound and a radiation sensitive substance on a substrate. Upon exposure of the material to radiation the metal layer is destroyed due to its chemical interaction with the products of the radiation-induced decomposition of the radiation sensitive substance in areas on which radiation, usually light, is incident. This provides a latent image which is subsequently developed to render it visible by utilising metal which remains intact to catalyst the selective decomposition of a metal compound deposited by conventional development or thermodeposition. This deposited metal constitutes a direct image. To produce the starting photographic material, however, it is necessary to achieve controlled evaporation and condensation on a substrate of metal and light sensitive substance in a definite ratio, which is generally a difficult matter to achieve. Moreover, the exposed material undergoes a particularly complicated treatment since it is necessary to achieve selective dissolution of the product of the reaction without affecting the latent metal image constituted by residual metal.
In a number of patent specifications, for example British patent specification No. 1,043,250, a method for the production of a metal image is described which utilises the ability of some metal oxides and sulphides to form a latent image on exposure to radiation, which image is formed of a material which catalyzes the deposition of a metal from the solution of a physical developer. A shortcoming of this method is that the latent image obtained disappears very quickly after illumination. Furthermore, the photographic material used is an emulsion and hence contains an organic substance as binder. Many of these materials are undesirable in some cases and their presence considerably complicates the production processes for the starting materials.
German Offenlegungsschriften Nos. 2,236,298 and 2,249,825 describe light sensitive materials which yield a metal image after exposure and suitable treatment. These photographic materials comprise halides or chalcogenides doped with metals and prepared in the form of an emulsion or thin evaporated layer. On exposure, the metal diffuses through the thickness of photographic material in the illuminated areas and the metal remaining on the non-illuminated areas is then removed by dissolution. Physical development is then effected to produce a metal image using the metal remaining in the photographic material as catalyst. As in British patent specifications Nos. 1,151,310 and 1,376,876 problems are encountered in the combining of two active materials, the light sensitive substance and the metal. In addition, a great part of the previously coated metal is lost when fixing the exposed material, so that the process is particularly expensive.
Finally, U.S. Pat. No. 3,707,372 describes an electromagnetic radiation sensitive element comprising a layer of inorganic material capable when exposed to electromagnetic actinic radiations of exhibiting physical and chemical characteristics different from those of the material not exposed to such radiations which material is inter alia an arsenic sulphide. These different properties may result from reaction of exposed material with a metal layer thereadjacent. Thus, when the element is exposed, for example to light, with an intensity and for a period of time sufficient to cause the different physical and chemical characteristics to appear, it becomes possible to utilize these different characteristics for specific purposes. In particular, differential solubility in a particular solvent may be used to remove either unexposed material or exposed material or different hydrophilic or oleophilic characteristics of the exposed and unexposed materials may be utilised in the production of a lithographic plate.